Rubber composition for use in tire treads

ABSTRACT

A rubber composition comprises a diene rubber including not less than 30% by weight of a modified conjugated diene polymer rubber and, per 100 parts by weight of the diene rubber, from 1 to 25 parts by weight of a tackifying resin, and from 25 to 80 parts by weight of a filler comprising not less than 50% by weight of silica. The modified conjugated diene polymer rubber has a terminal modified group comprising a functional group that interacts with silica. In the modified conjugated diene polymer rubber, aromatic vinyl unit content is from 38% to 48% by weight, vinyl unit content is from 20% to 35%, weight-average molecular weight is from 600,000 to 1,000,000, and glass transition temperature is from −22 to −32° C. A glass transition temperature of the tackifying resin is from 50 to 110° C. higher than the glass transition temperature of the modified conjugated diene polymer rubber.

TECHNICAL FIELD

The present technology relates to a rubber composition for use in tiretreads and particularly relates to a rubber composition for use in tiretreads that enhances low rolling resistance and wet performance.

BACKGROUND ART

Increased interest in the global environmental issues has led to ademand for superior fuel consumption performance in pneumatic tires,along with steering stability and braking performance when traveling onwet road surfaces, and superior safety performance. As a result, bycompounding silica in rubber compositions that form tread portions,dynamic visco-elasticity characteristics of the tread rubber such asloss tangent (tan δ) and the like has been improved, heat build-up hasbeen suppressed, rolling resistance reduced, and fuel consumptionperformance improved, which has led to enhancements in wet performance.However, silica has poor affinity with diene rubber and dispersibilitytends to be insufficient. Particularly, when the particle diameter ofthe silica is small, dispersibility worsens and, as a result, theeffects of achieving reduced heat build-up and improving wet performancehave not been obtainable.

To resolve this problem, Japanese Unexamined Patent ApplicationPublication No. 2009-091498 proposes improving the dispersibility ofsilica by compounding silica in a rubber composition with aterminal-modified solution polymerization styrene butadiene rubber wherethe terminals are modified by a polyorganosiloxane or the like, therebyreducing heat build-up (tan δ at 60° C.) and enhancing wet gripperformance (tan δ at 0° C.). Additionally, Japanese Unexamined PatentApplication Publication No. 2007-321046 proposes a rubber compositioncomprising a styrene-butadiene copolymer rubber and, per 100 parts byweight thereof, from 80 to 180 parts by weight of a filler comprisingnot less than 50 parts by weight of a silica, and from 5 to 60 parts byweight of a resin having a softening point of 100 to 150° C.

However, the rolling resistance and wet performance obtained by therubber compositions proposed in Japanese Unexamined Patent ApplicationPublication No. 2009-091498 and Japanese Unexamined Patent ApplicationPublication No. 2007-321046 are below those demanded by users and thereis a need for further improvement in low rolling resistance and wetperformance.

SUMMARY

The present technology provides a rubber composition for use in tiretreads by which low rolling resistance and wet performance can beenhanced to or beyond conventional levels.

A rubber composition for use in tire treads comprises a diene rubberincluding not less than 30% by weight of a modified conjugated dienepolymer rubber and, per 100 parts by weight of the diene rubber, from 1to 25 parts by weight of a tackifying resin, and from 25 to 80 parts byweight of a filler. The filler comprises not less than 50% by weight ofsilica. Moreover, the modified conjugated diene polymer rubber isobtained by copolymerizing a conjugated diene monomer unit and anaromatic vinyl monomer unit in a hydrocarbon solvent using an organicactive metal compound as an initiator. A resulting active conjugateddiene polymer chain thereof has a terminal modified group, obtained byreacting the active terminal of the polymer chain with at least one typeof compound having a functional group that is reactable with the activeterminal of the polymer chain. The terminal modified group has afunctional group that interacts with the silica. Furthermore, in themodified conjugated diene polymer rubber, aromatic vinyl unit content isfrom 38% to 48% by weight, vinyl unit content is from 20% to 35%,weight-average molecular weight is from 600,000 to 1,000,000, and glasstransition temperature is from −22 to −32° C. A glass transitiontemperature of the tackifying resin is from 50 to 110° C. higher thanthe glass transition temperature of the modified conjugated dienepolymer rubber.

The rubber composition for use in tire treads of the present technologycomprises the diene rubber comprising not less than 25% by weight of themodified conjugated diene polymer rubber in which the aromatic vinylunit content is from 38% to 48% by weight, the vinyl unit content isfrom 20% to 35%, the weight-average molecular weight is from 600,000 to1,000,000, and the glass transition temperature is from −22 to −32° C.The active conjugated diene polymer chain is obtained by copolymerizingthe conjugated diene monomer unit and the aromatic vinyl monomer unit.The active terminal of the active conjugated diene polymer chain isreacted with at least one type of compound having the functional groupthat is reactable with the active terminal in order to form the terminalmodified group. The terminal modified group has the functional groupthat interacts with silica. Additionally, the rubber composition for usein tire treads of the present technology comprises, per 100 parts byweight of the diene rubber, from 1 to 25 parts by weight of thetackifying resin and from 25 to 80 parts by weight of the fillercomprising not less than 50% by weight of silica. As a result, affinitybetween the diene rubber and the silica is increased and thedispersibility of the silica is enhanced, leading to a reduction in heatbuild-up, a decline in rolling resistance, and an enhancement in wetperformance. Particularly, the modified conjugated diene polymer rubbertakes on a fine phase-separated form due to setting the aromatic vinylunit content to from 38% to 48% by weight. Moreover, the terminalmodified group, produced through the reaction of the active terminal ofthe active conjugated diene polymer chain and the at least one compoundhaving the functional group that is reactable with the active terminalof the active conjugated diene polymer chain, has the functional groupthat interacts with the silica and the weight-average molecular weightof the modified conjugated diene polymer rubber is set to from 600,000to 1,000,000, resulting in the concentration of the terminal modifiedgroup being made appropriate. Therefore, the terminal modified groupacts effectively on the silica, the dispersibility of the silica isfurther improved, the rolling resistance of the pneumatic tire issignificantly reduced, and the wet performance can be further enhanced.Furthermore, the glass transition temperature of the tackifying resin isset to be from 50 to 110° C. higher than the glass transitiontemperature of the modified conjugated diene polymer rubber and, as aresult, the wet performance can be further enhanced while maintainingsuperior low rolling resistance.

The compound comprising the functional group that is reactable with theactive terminal of the active conjugated diene polymer chain describedabove preferably comprises at least one type of polyorganosiloxanecompound selected from general formulae (I) to (III) below.

In formula (I), R¹ to R⁸ are identical or different and are alkyl groupshaving from 1 to 6 carbons or aryl groups having from 6 to 12 carbons;X¹ and X⁴ are identical or different and are groups having functionalgroups that react with the active terminal of the active conjugateddiene polymer chain, alkyl groups having from 1 to 6 carbons, or arylgroups having from 6 to 12 carbons; X² is a group having a functionalgroup that reacts with the active terminal of the active conjugateddiene polymer chain; X³ is a group including from 2 to 20 repeatingalkylene glycol units, a portion of the X³ moieties optionally beinggroups derived from groups including from 2 to 20 repeating alkyleneglycol units; and m is an integer from 3 to 200, n is an integer from 0to 200, and k is an integer from 0 to 200.

In formula (II), R⁹ to R¹⁶ are identical or different and are alkylgroups having from 1 to 6 carbons or aryl groups having from 6 to 12carbons; and X⁵ to X⁸ are groups having functional groups that reactwith the active terminal of the active conjugated diene polymer chain.

In formula (III), R¹⁷ to R¹⁹ are identical or different and are alkylgroups having from 1 to 6 carbons or aryl groups having from 6 to 12carbons; and X⁹ to X¹¹ are groups having functional groups that reactwith the active terminal of the active conjugated diene polymer chain;and s is an integer from 1 to 18.

The low rolling resistance and the wet performance can be enhanced to orbeyond conventional levels via a pneumatic tire in which the rubbercomposition described above is used in the tread portion.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partial cross-sectional view in a tire meridian directionillustrating an example of an embodiment of a pneumatic tire in which arubber composition for use in tire treads of the present technology isused.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 illustrates an example of an embodiment of a pneumatic tire inwhich a rubber composition for use in tire treads is used. In FIG. 1, 1is a tread portion, 2 is a side wall portion, and 3 is a bead portion.

In FIG. 1, two layers of a carcass layer 4, formed by arrangingreinforcing cords extending in a tire radial direction in a tirecircumferential direction at a predetermined pitch and embedding thesereinforcing cords in a rubber layer, is disposed extending between leftand right side bead portions 3. Both ends are made to sandwich a beadfiller 6 around a bead core 5 that is embedded in the bead portions 3and are folded back in a tire axial direction from the inside to theoutside. An inner liner layer 7 is disposed inward of the carcass layer4. Two layers of a belt layer 8, formed by arranging reinforcing cordsextending inclined to the tire circumferential direction in the tireaxial direction at a predetermined pitch and embedding these reinforcingcords in a rubber layer, is disposed on an outer circumferential side ofthe carcass layer 4 of the tread portion 1. The reinforcing cords of thetwo layers of a belt layer 8 cross interlamilarly so that the inclinedirections with respect to the tire circumferential direction areopposite each other. A belt cover layer 9 is disposed on an outercircumferential side of the belt layers 8. The tread portion 1 is formedfrom a tread rubber layer 12 on an outer circumferential side of thebelt cover layer 9. The tread rubber layer 12 is formed from the rubbercomposition for use in tire treads. A side rubber layer 13 is disposedoutward of the carcass layer 4 in each side wall portion 2, and a rimcushion rubber layer 14 is provided outward of the portion of thecarcass layer 4 that is folded back around each of the bead portions 3.

In the rubber composition for use in tire treads of the presenttechnology, the rubber component is a diene rubber and the diene rubbernecessarily comprises a modified conjugated diene polymer rubber. Themodified conjugated diene polymer rubber is a conjugated diene polymerrubber produced by solution polymerization, configured to havefunctional groups at both terminals of the molecular chain. Bycompounding the modified conjugated diene polymer rubber, affinity withsilica is increased, and dispersibility is improved. As a result, theeffects of the silica are further enhanced and the low rollingresistance and the wet performance are improved.

In the present technology, the backbone of the modified conjugated dienepolymer is formed by a copolymer obtained by copolymerizing a conjugateddiene monomer unit and an aromatic vinyl monomer unit. Examples of theconjugated diene monomer unit include 1,3-butadiene,isoprene(2-methyl-1,3-butadiene), 2,3-dimethyl-1,3-butadiene,2-chloro-1,3-butadiene, 1,3-pentadiene, and the like. Examples of thearomatic vinyl monomer unit include styrene, 2-methylstyrene,3-methylstyrene, 4-methylstyrene, alpha-methylstyrene,2,4-dimethylstyrene, 2,4-diisoisopropylstyrene, 4-tert-butylstyrene,divinylbenzene, tert-butoxystyrene, vinylbenzyldimethylamine,(4-vinylbenzyl)dimethylaminoethyl ether, N,N-dimethyl aminoethylstyrene,vinyl pyridine, and the like.

The terminals of the conjugated diene polymer backbone are preferablyformed from isoprene unit blocks. As a result of the terminals beingformed from isoprene unit blocks, when the terminals are modified andthe silica is compounded, affinity between the modified conjugated dienepolymer and the silica is excellent and reduced heat build-up and wetperformance are also excellent. Thus, in cases where the conjugateddiene monomer units forming the polymer comprise conjugated dienes otherthan isoprene units, isoprene unit blocks are preferably introduced onthe polymer terminals by adding isoprene to a solution containing thepolymer having an active terminal prior to adding the compound havingthe functional group that is reactable with the active terminal of theactive conjugated diene polymer chain or, alternatively, betweensubsequent adding of portions of this compound.

In the present technology, the conjugated diene polymer is prepared bycopolymerizing the conjugated diene monomer unit and the aromatic vinylmonomer unit described above in a hydrocarbon solvent, using an organicactive metal compound as an initiator. It is sufficient that thehydrocarbon solvent be a commonly used solvent, and examples thereofinclude cyclohexane, n-hexane, benzene, toluene, and the like.

The organic active metal catalyst to be used is preferably an organicalkali metal compound, and examples thereof include organic monolithiumcompounds such as n-butyllithium, sec-butyllithium, t-butyllithium,hexyl lithium, phenyl lithium, stilbene lithium, and the like; organicpolyhydric lithium compounds such as dilithiomethane,1,4-dilithiobutane, 1,4-dilithio-2-ethylcyclohexane,1,3,5-trilithiobenzene, and the like; organic sodium compounds such assodium napthalene and the like; and organic potassium compounds such aspotassium napthalene and the like. Additionally,3,3-(N,N-dimethylamino)-1-propyl lithium, 3-(N,N-diethylamino)-1-propyllithium, 3-(N,N-dipropylamino)-1-propyl lithium, 3-morpholino-1-propyllithium, 3-imidazole-1-propyl lithium, and organic lithium compoundshaving their chains extended by 1 to 10 units of butadiene, isoprene, orstyrene; and the like can be used.

In the polymerization reaction, a polar aprotic compound such as anether such as diethylether, diethylene glycol dimethylether,tetrahydrofuran, 2,2-bis(2-oxolanyl)propane, and the like, or an aminesuch as triethylamine, tetramethyl ethylenediamine, and the like mayalso be added for the purpose of randomly copolymerizing the aromaticvinyl monomer units and the conjugated diene monomer units.

In the present technology, at least one type of compound having areactable functional group is attached to the active terminal of theactive conjugated diene polymer chain obtained by copolymerizing theconjugated diene monomer units and the aromatic vinyl monomer units,and, thereby, a terminal modified group is produced. In this case, it issufficient that the compound having the reactable functional group atthe active terminal of the active conjugated diene polymer chain beattached to at least one active conjugated diene polymer chain, and oneor more active conjugated diene polymer chains can be attached to eachcompound. That is, the modified conjugated diene polymer rubber used inthe present technology can include modified rubbers having modifyinggroups at both terminals of the conjugated diene polymer, modifiedrubbers in which one or more of the modifying groups is optionallyattached to a different conjugated diene polymer, and mixtures of aplurality of these modified rubbers. Additionally, the reaction betweenthe active terminal of the active conjugated diene polymer chain and thecompound having the functional group that is reactable with the activeterminal can be a single-stage or multiple-stage reaction. Moreover, anidentical or different compound may be sequentially reacted.

In the present technology, examples of the compound having thefunctional group that is reactable with the active terminal of theactive conjugated diene polymer chain include tin compounds, siliconcompounds, silane compounds, amido compounds and/or imide compounds,isocyanate and/or isothiocyanate compounds, ketone compounds, estercompounds, vinyl compounds, oxirane compounds, thiirane compounds,oxetane compounds, polysulfide compounds, polysiloxane compounds,polyorganosiloxane compounds, polyether compounds, polyene compounds,halogen compounds, and compounds having fullerenes. Among these,polyorganosiloxane compounds are preferable. One of these compounds orcombinations of a plurality of these compounds can be attached to thepolymer.

Specific examples of the compound that is reactable with the activeterminal of the active conjugated diene polymer chain includepolyglycidyl ethers of polyhydric alcohol such as ethylene glycoldiglycidyl ether, glycerin triglycidyl ether, and the like; polyglycidylethers of aromatic compounds having two or more phenol groups such asbisphenol A diglycidylate and the like; polyepoxy compounds such as1,4-diglycidyl benzene, 1,3,5-triglycidyl benzene, liquid polybutadienepolyepoxydate, and the like; epoxy group-containing tertiary amines suchas 4,4′-diglycidyl-diphenyl methylamine, 4,4′-diglycidyl-dibenzylmethylamine, and the like; diglycidyl amino compounds such as diglycidylaniline, diglycidyl-o-toluidine, tetraglycidyl metaxylylene diamine,tetraglycidyl amino diphenylmethane, tetraglycidyl-p-phenylenediamine,diglycidyl amino methylcyclohexane, tetraglycidyl-1,3-bis aminomethylcyclohexane, and the like; and the like.

Examples of the silicon compound include tetrachlorosilicon,tetrabromosilicon, methyltrichlorosilicon, butyltrichlorosilicon,dichlorosilicon, bis(trichlorosilyl)silicon, and the like.

Examples of the tin compound include tetrachlorostannate,tetrabromostannate, methyltrichlorostannate, butyltrichlorostannate,dichlorostannate, bis(trichlorosilyl)stannate, and the like.

Examples of the silane compound include silane compounds having at leastone selected from an alkoxy group, a phenoxy group, and a halogen.Examples of such silane compounds include dimethoxy dimethylsilane,diphenoxy dimethylsilane, diethoxy diethylsilane, triphenoxymethylsilane, triphenoxy vinylsilane, trimethoxy vinylsilane, triethoxyvinylsilane, tri(2-methylbutoxy)ethylsilane,tri(2-methylbutoxy)vinylsilane, triphenoxy phenylsilane,tetraphenoxysilane, tetraethoxysilane, tetramethoxysilane,tetrakis(2-ethylhexyloxy)silane, phenoxydivinyl chlorosilane,methoxybiethyl chlorosilane, diphenoxymethyl chlorosilane,diphenoxyphenyl iodosilane, diethoxymethyl chlorosilane, dimethoxymethylchlorosilane, trimethoxy chlorosilane, triethoxy chlorosilane,triphenoxy chlorosilane, tris(2-ethylhexyloxy)chlorosilane,phenoxymethyl dichlorosilane, methoxyethyl dichlorosilane, ethoxymethyldichlorosilane, phenoxyphenyl diiodosilane, diphenoxy dichlorosilane,dimethoxy dichlorosilane, bis(2-methylbutoxy)dibromosilane,bis(2-methylbutoxy)dichlorosilane, diethoxy dichlorosilane, methoxytrichlorosilane, ethoxy trichlorosilane, phenoxy trichlorosilane,(2-ethylhexyloxy)trichlorosilane, (2-methylbutoxy)trichlorosilane, andthe like.

Additionally, aside from the functional groups described above, thesilane compound can have a glycidyl group, an epoxy group, amethacryloxy group, and the like. Examples of such silane compoundsinclude γ-glycidoxyethyl trimethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxybutyl trimethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropyl tripropoxysilane, γ-glycidoxypropyltributoxysilane, γ-glycidoxypropyl triphenoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ-glycidoxypropyl ethyldimethoxysilane,γ-glycidoxypropyl ethyldiethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyl methyldipropoxysilane,γ-glycidoxypropyl methyldibutoxysilane, γ-glycidoxypropylmethyldiphenoxysilane, γ-glycidoxypropyl dimethylmethoxysilane,γ-glycidoxypropyl diethylethoxysilane, γ-glycidoxypropyldimethylethoxysilane, γ-glycidoxypropyl dimethylphenoxysilane,γ-glycidoxypropyl diethylmethoxysilane, γ-glycidoxypropylmethyldiisopropeneoxysilane, bis(γ-glycidoxypropyl)dimethoxysilane,bis(γ-glycidoxypropyl)diethoxysilane,bis(γ-glycidoxypropyl)dipropoxysilane,bis(γ-glycidoxypropyl)dibutoxysilane,bis(γ-glycidoxypropyl)diphenoxysilane,bis(γ-glycidoxypropyl)methylmethoxysilane,bis(γ-glycidoxypropyl)methylethoxysilane,bis(γ-glycidoxypropyl)methylpropoxysilane,bis(γ-glycidoxypropyl)methylbutoxysilane,bis(γ-glycidoxypropyl)methylphenoxysilane,tris(γ-glycidoxypropyl)methoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyl triethoxysilane,γ-methacryloxymethyl trimethoxysilane, γ-methacryloxyethyltriethoxysilane, bis(γ-methacryloxypropyl)dimethoxysilane,tris(γ-methacryloxypropyl)methoxysilane,β-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane,β-(3,4-epoxycyclohexyl)ethyl-triethoxysilane,β-(3,4-epoxycyclohexyl)ethyl-tripropoxysilane,β-(3,4-epoxycyclohexyl)ethyl-tributoxysilane,β-(3,4-epoxycyclohexyl)ethyl-triphenoxysilane,β-(3,4-epoxycyclohexyl)propyl-trimethoxysilane,β-(3,4-epoxycyclohexyl)ethyl-methyldimethoxysilane,β-(3,4-epoxycyclohexyl)ethyl-ethyldimethoxysilane,β-(3,4-epoxycyclohexyl)ethyl-ethyldiethoxysilane,β-(3,4-epoxycyclohexyl)ethyl-methyldiethoxysilane,β-(3,4-epoxycyclohexyl)ethyl-methyldipropoxysilane,β-(3,4-epoxycyclohexyl)ethyl-methyldibutoxysilane,β-(3,4-epoxycyclohexyl)ethyl-methyldiphenoxysilane,β-(3,4-epoxycyclohexyl)ethyl-dimethylmethoxysilane,β-(3,4-epoxycyclohexyl)ethyl-diethylethoxysilane,β-(3,4-epoxycyclohexyl)ethyl-dimethylethoxysilane,β-(3,4-epoxycyclohexyl)ethyl-dimethylpropoxysilane,β-(3,4-epoxycyclohexyl)ethyl-dimethylbutoxysilane,β-(3,4-epoxycyclohexyl)ethyl-dimethylphenoxysilane,β-(3,4-epoxycyclohexyl)ethyl-diethylmethoxysilane,β-(3,4-epoxycyclohexyl)ethyl-methyldiisopropeneoxysilane, and the like.

Examples of the isocyanate compound or isothiocyanate compound includearomatic polyisocyanate compounds such as 2,4-tolylene diisocyanate,2,6-tolylene diisocyanate, diphenylmethane diisocyanate, naphthalenediisocyanate, tolidine diisocyanate, triphenylmethane triisocyanate,p-phenylene diisocyanate, tris(isocyanatephenyl)thiophosphate, xylylenediisocyanate, benzene-1,2,4-triisocyanate,naphthalene-1,2,5,7-tetraisocyanate, naphthalene-1,3,7-triisocyanate,phenylisocyanate, hexamethylene diisocyanate, methylcyclohexanediisocyanate, phenyl-1,4-diisothiocyanate, 2,4-tolylene diisocyanate,diphenylmethane diisocyanate, naphthalene diisocyanate, and the like.

Further examples include N-substituted aminoketones such as4-dimethylamino benzophenone, 4-diethylamino benzophenone,4-di-t-butylamino benzophenone, 4-diphenylamino benzophenone,4,4′-bis(dimethylamino)benzophenone, 4,4′-bis(diethylamino)benzophenone,4,4′-bis(di-t-butylamino)benzophenone,4,4′-bis(diphenylamino)benzophenone, 4,4′-bis(divinylamino)benzophenone,4-dimethylamino acetophenone, 4-diethylamino acetophenone,1,3-bis(diphenylamino)-2-propanone,1,7-bis(methylethylamino)-4-heptanone, and the like and correspondingN-substituted aminothioketones; N-substituted aminoaldehydes such as4-diethylamino benzaldehyde, 4-divinylamino benzaldehyde, and the likeand corresponding N-substituted aminothioaldehydes; N-substitutedlactams such as N-methyl-β-propiolactam, N-t-butyl-β-propiolactam,N-phenyl-β-propiolactam, N-methoxyphenyl-β-propiolactam,N-naphthyl-β-propiolactam, N-methyl-2-pyrrolidone,N-t-butyl-2-pyrrolidone, N-phenyl-pyrrolidone,N-methoxyphenyl-2-pyrrolidone, N-vinyl-2-pyrrolidone,N-benzyl-2-pyrrolidone, N-naphthyl-2-pyrrolidone,N-methyl-5-methyl-2-pyrrolidone, N-methyl-3,3′-dimethyl-2-pyrrolidone,N-t-butyl-3,3′-dimethyl-2-pyrrolidone,N-phenyl-3,3′-dimethyl-2-pyrrolidone, N-methyl-2-piperidone,N-t-butyl-2-piperidone, N-phenyl-piperidone,N-methoxyphenyl-2-piperidone, N-vinyl-2-piperidone,N-benzyl-2-piperidone, N-naphthyl-2-piperidone,N-methyl-3,3′-dimethyl-2-piperidone,N-phenyl-3,3′-dimethyl-2-piperidone, N-methyl-ε-caprolactam,N-phenyl-ε-caprolactam, N-methoxyphenyl-ε-caprolactam,N-vinyl-ε-caprolactam, N-benzyl-ε-caprolactam, N-naphthyl-ε-caprolactam,N-methyl-ω-laurilolactam, N-phenyl-ω-laurilolactam,N-t-butyl-laurilolactam, N-vinyl-ω-laurilolactam,N-benzyl-ω-laurilolactam, and the like and corresponding thiolactams;N-substituted ethyleneureas such as 3-dimethyl-2-imidazolidinone,1,3-diethyl-2-imidazolidinone, 1,3-dipropyl-2-imidazolidinone,1-methyl-3-ethyl-2-imidazolidinone, 1-methyl-3-propyl-2-imidazolidinone,1-methyl-3-methyl-2-imidazolidinone,1-methyl-3-2-ethoxyethyl-2-imidazolidinone,1,3-dimethyl-3,4,5,6-tetrahydropyrimidinone, and the like andcorresponding N-substituted thioethyleneureas and the like;benzophenones and thiobenzophenones having at least one amino group,alkylamino group or dialkylamino group on one or both benzene rings suchas 4,4′-bis(dimethylamino)-benzophenone,4,4′-bis(diethylamino)-benzophenone,4,4′-bis(dibutylamino)-benzophenone, 4,4′-diamino benzophenone,4-dimethylamino benzophenone, and the like and correspondingthiobenzophenones and the like; and the like.

The halogen and/or alkoxy group-containing silicon compound preferablyis a compound expressed by general formula (IV) below. A plurality ofactive conjugated diene polymer chains can easily be attached to amolecule of this compound.

In formula (IV), X¹ and X² are halogen atoms or alkoxy groups havingfrom 1 to 20 carbons. p and q are each independently integers from 0 to3 and the total number halogen atoms and alkoxy groups having from 1 to20 carbons in the compound expressed by formula (IV) is not less than 5.R¹ and R² are each monovalent hydrocarbon groups having from 1 to 20carbons. n is an integer of from 0 to 20 and A¹ and A² are eachindependently divalent hydrocarbons having a single bond or from 1 to 20carbons. A³ is a divalent group expressed by the formula —(SiX³ _(r)A³_(2-r))_(m)—, —NR⁴—, or —N(-A⁴-SiX⁴ _(s)R⁵ _(3-s))—. X³ and X⁴ arehalogen atoms or alkoxy groups having from 1 to 20 carbons. R³ and R⁵are monovalent hydrocarbon groups having from 1 to 20 carbons. R⁴ is ahydrogen atom or a monovalent hydrocarbon group having from 1 to 20carbons. A⁴ is a divalent hydrocarbon group having a single bond or from1 to 20 carbons. r is an integer of from 0 to 2 and m is an integer offrom 0 to 20. s is an integer of from 0 to 3.

Examples of the compound expressed by general formula (IV) includehalogenated silicon compounds such as hexachlorodisilane,bis(trichlorosilyl)methane, 1,2-bis(trichlorosilyl)ethane,1,3-bis(trichlorosilyl)propane, 1,4-bis(trichlorosilyl)butane,1,5-bis(trichlorosilyl)pentane, 1,6-bis(trichlorosilyl)hexane, and thelike; alkoxysilane compounds such as hexamethoxydisilane,hexaethoxydisilane, bis(trimethoxysilyl)methane,bis(triethoxysilyl)methane, bis(trimethoxysilyl)ethane,bis(triethoxysilyl)ethane, bis(trimethoxysilyl)propane,bis(triethoxysilyl)propane, bis(trimethoxysilyl)butane,bis(triethoxysilyl)butane, bis(trimethoxysilyl)heptane,bis(triethoxysilyl)heptane, bis(trimethoxysilyl)hexane,bis(triethoxysilyl)hexane, bis(trimethoxysilyl)benzene,bis(triethoxysilyl)benzene, bis(trimethoxysilyl)cyclohexane,bis(triethoxysilyl)cyclohexane, bis(triethoxysilyl)benzene,bis(trimethoxysilyl)octane, bis(triethoxysilyl)octane,bis(trimethoxysilyl)nonane, bis(triethoxysilyl)nonane,bis(trimethoxysilyl)ethylene, bis(triethoxysilyl)ethylene,bis(trimethoxysilylethyl)benzene, bis(triethoxysilylethyl)benzene,bis(3-trimethoxysilylpropyl)ethane, bis(3-triethoxysilylpropyl)ethane,and the like; alkoxysilane compounds having an amino group such asbis(3-trimethoxysilylpropyl)methylamine,bis(3-triethoxysilylpropyl)methylamine,bis(3-trimethoxysilylpropyl)ethylamine,bis(3-triethoxysilylpropyl)ethylamine,bis(3-trimethoxysilylpropyl)propylamine,bis(3-triethoxysilylpropyl)propylamine,bis(3-trimethoxysilylpropyl)butylamine,bis(3-triethoxysilylpropyl)butylamine,bis(3-trimethoxysilylpropyl)phenylamine,bis(3-triethoxysilylpropyl)phenylamine,bis(3-trimethoxysilylpropyl)benzylamine,bis(3-triethoxysilylpropyl)benzylamine,bis(trimethoxysilylmethyl)methylamine,bis(triethoxysilylmethyl)methylamine,bis(2-trimethoxysilylethyl)methylamine,bis(2-triethoxysilylethyl)methylamine,bis(triethoxysilylmethyl)propylamine,bis(2-triethoxysilylethyl)propylamine, and the like; alkoxysilanecompounds having an amino group such astris(trimethoxysilylmethyl)amine, tris(2-triethoxysilylethyl)amine,tris(3-trimethoxysilylpropyl)amine, tris(3-triethoxysilylpropyl)amine,and the like; and the like.

The polyorganosiloxane compound is preferably a compound expressed bygeneral formulae (I) to (III) below. That is, the compound having thefunctional group that is reactable with the active terminal of theactive conjugated diene polymer chain preferably includes at least onecompound selected from these polyorganosiloxane compounds, and mayinclude a combination of a plurality of these compounds. Additionally,these polyorganosiloxane compounds may be combined with another compoundhaving a functional group that is reactable with the active terminal(e.g. the compound expressed by formula (IV) above).

In formula (I), R¹ to R⁸ are identical or different and are alkyl groupshaving from 1 to 6 carbons or aryl groups having from 6 to 12 carbons;X¹ and X⁴ are identical or different and are groups having functionalgroups that react with the active terminal of the active conjugateddiene polymer chain, alkyl groups having from 1 to 6 carbons, or arylgroups having from 6 to 12 carbons; X² is a group having a functionalgroup that reacts with the active terminal of the active conjugateddiene polymer chain; X³ is a group including from 2 to 20 repeatingalkylene glycol units, a portion of the X³ moieties optionally beinggroups derived from groups including from 2 to 20 repeating alkyleneglycol units; and m is an integer from 3 to 200, n is an integer from 0to 200, and k is an integer from 0 to 200.

In formula (II), R⁹ to R¹⁶ are identical or different and are alkylgroups having from 1 to 6 carbons or aryl groups having from 6 to 12carbons; and X⁵ to X⁸ are groups having functional groups that reactwith the active terminal of the active conjugated diene polymer chain.

In formula (III), R¹⁷ to R¹⁹ are identical or different and are alkylgroups having from 1 to 6 carbons or aryl groups having from 6 to 12carbons; and X⁹ to X¹¹ are groups having functional groups that reactwith the active terminal of the active conjugated diene polymer chain;ands is an integer from 1 to 18.

Examples of the alkyl groups having from 1 to 6 carbons that constitutethe R¹ to R⁸, X¹, and X⁴ moieties in the polyorganosiloxane expressed bygeneral formula i (I) above include methyl groups, ethyl groups,n-propyl groups, isopropyl groups, butyl groups, pentyl groups, hexylgroups, cyclohexyl groups, and the like. Examples of the aryl groupshaving from 6 to 12 carbons include phenyl groups, methylphenyl groups,and the like. Among these alkyl groups and aryl groups, methyl groupsare particularly preferable.

Examples of the group having the functional group, which reacts with theactive terminal of the polymer chain, constituting the X¹, X², and X⁴moieties in the polyorganosiloxane expressed by general formula (I)include alkoxyl groups having from 1 to 5 carbons, hydrocarbon groupscontaining a 2-pyrrolidonyl group, and epoxy group-containing groupshaving from 4 to 12 carbons.

Examples of the alkoxyl groups having from 1 to 5 carbons constitutingthe X¹, X², and X⁴ moieties include methoxy groups, ethoxy groups,propoxy groups, isopropoxy groups, butoxy groups, and the like. Amongthese methoxy groups are preferable. In cases where at least one of theX¹, X², and X⁴ moieties is the alkoxyl group having from 1 to 5 carbons,when the polyorganosiloxane having the alkoxyl group is reacted with theactive terminal of the active conjugated diene polymer chain, linkagebetween the silicon atom and the oxygen atom of the alkoxyl group breaksand the active conjugated diene polymer chain attaches directly to thesilicon atom, thus forming a single bond.

Preferable examples of the hydrocarbon group containing a 2-pyrrolidonylgroup constituting the X¹, X², and X⁴ moieties include the groupsexpressed by the general formula (V) below.

In formula (V), j is an integer of from 2 to 10, and j is particularlypreferably 2.

Thus, when the polyorganosiloxane, in which at least one of the X¹, X²,and X⁴ moieties comprises the hydrocarbon group containing the2-pyrrolidonyl group, is reacted with the active terminal of the activeconjugated diene polymer chain, the carbon-oxygen bond in the carbonylgroup constituting the 2-pyrrolidonyl group breaks and a structure isformed in which the polymer chain is bonded to the carbon atom.

Preferable examples of the epoxy group-containing group having from 4 to12 carbons constituting the X¹, X², and X⁴ moieties include the groupsexpressed by the general formula (VI) below.

General Formula (VI): ZYE

In formula (VI), Z is an alkylene group or an alkyl arylene group havingfrom 1 to 10 carbons; Y is a methylene group, a sulfur atom, or anoxygen atom; and E is an epoxy group-containing hydrocarbon group havingfrom 2 to 10 carbons. Among these, preferably Y is an oxygen atom; morepreferably Y is an oxygen atom and E is a glycidyl group; and even morepreferably Z is an alkylene group having three carbons, Y is an oxygenatom, and E is a glycidyl group.

In the polyorganosiloxane expressed by general formula (I), in caseswhere at least one of the X¹, X², and X⁴ moieties is an epoxygroup-containing group having from 4 to 12 carbons, when thepolyorganosiloxane is reacted with the active terminal of the activeconjugated diene polymer chain, the carbon-oxygen bond forming the epoxyring breaks and a structure is formed in which the polymer chain isbonded to the carbon atom.

In the polyorganosiloxane expressed by general formula (I), of theabove, X¹ and X⁴ are preferably epoxy group-containing groups havingfrom 4 to 12 carbons or alkyl group having from 1 to 6 carbons.Additionally, X² is preferably an epoxy group-containing group havingfrom 4 to 12 carbons.

In the polyorganosiloxane expressed by general formula (I), X³ is agroup including from 2 to 20 repeating alkylene glycol units. Preferableexamples of the group including from 2 to 20 repeating alkylene glycolunits include the group expressed by general formula (VII) below.

In formula (VII), t is an integer of from 2 to 20, R¹ is an alkylenegroup or an alkyl arylene group having from 2 to 10 carbons, R³ is ahydrogen atom or a methyl group, and R² is an alkoxyl group or anaryloxy group having from 1 to 10 carbons. Among these, preferably, t isan integer of from 2 to 8, R¹ is an alkylene group having three carbons,R³ is a hydrogen atom, and R² is a methoxy group.

In the polyorganosiloxane expressed by general formula (II), R⁹ to R¹⁶are identical or different and are alkyl groups having from 1 to 6carbons or aryl groups having from 6 to 12 carbons. X⁵ to X⁸ are groupshaving functional groups that react with the active terminal of thepolymer chain.

In the polyorganosiloxane expressed by general formula (III), R¹⁷ to R¹⁹are identical or different and are alkyl groups having from 1 to 6carbons or aryl groups having from 6 to 12 carbons. X⁹ to X¹¹ are groupshaving functional groups that react with the active terminal of thepolymer chain. s is an integer from 1 to 18.

In the polyorganosiloxane expressed by general formula (II) and generalformula (III) above, the alkyl group having from 1 to 6 carbons, thearyl group having from 6 to 12 carbons, and the group having thefunctional group that reacts with the active terminal of the polymerchain are synonymous with those recited for the polyorganosiloxaneexpressed by general formula (I).

Furthermore, the terminal modified group produced as a result of thereaction described above has a functional group that interacts withsilica. This functional group that interacts with silica may be thefunctional group included in the structure of the compound describedabove. The functional group may also be a functional group that isobtained as a result of the reaction between the compound and the activeterminal. The functional group that interacts with silica is notparticularly limited, and examples thereof include an alkoxysilyl group,a hydroxyl group (including those having organosiloxane structures), analdehyde group, a carboxyl group, an amino group, an imino group, anepoxy group, an amido group, a thiol group, an ether group, and thelike. Among these, the hydroxyl group (including that having anorganosiloxane structure) is preferable. Thus, the terminal modifiedgroup includes the functional group that interacts with silica and,therefore, affinity with silica is further enhanced, which leads tosignificant improvement in dispersibility.

In the present technology, the concentration of the terminal modifiedgroup in the modified conjugated diene polymer rubber is determined byits relationship to the weight-average molecular weight (Mw) of themodified conjugated diene polymer rubber. The weight-average molecularweight of the modified conjugated diene polymer rubber is from 600,000to 1,000,000 and is preferably from 650,000 to 850,000. If theweight-average molecular weight of the modified conjugated diene polymerrubber is less than 600,000, the concentration of the terminal modifiedgroup of the modified conjugated diene polymer rubber will increase and,while the dispersibility of the silica in the rubber composition will bebetter, the molecular weight of the polymer itself will be low.Therefore, the effects of improving the strength and rigidity of therubber composition will not be obtained and, moreover, the degree ofimprovement in viscoelastic characteristics will be limited. If theweight-average molecular weight of the modified conjugated diene polymerrubber exceeds 1,000,000, the concentration of the terminal modifiedgroup of the modified conjugated diene polymer rubber will decrease,affinity with the silica will be insufficient, and dispersibility willbe negatively affected. As a result, the effects of reducing the rollingresistance will be insufficient and the wet performance will beinsufficient. Additionally, at the same time, the rigidity and thestrength of the rubber composition will decline. Note that theweight-average molecular weight (Mw) of the modified conjugated dienepolymer rubber is measured via gel permeation chromatography (GPC), interms of standard polystyrene.

An aromatic vinyl unit content in the modified conjugated diene polymerrubber used in the present technology is from 38% to 48% by weight andpreferably from 40% to 45% by weight. By configuring the aromatic vinylunit content in the modified conjugated diene polymer rubber to bewithin this range, the rigidity and the strength of the rubbercomposition can be increased and the wet resistance can be furtherenhanced when the rubber is formed into a pneumatic tire. Whencompounding a diene rubber other than the modified conjugated dienepolymer rubber, the modified conjugated diene polymer rubber takes on afine phase-separated form from the other diene rubber. As a result, themodified conjugated diene polymer rubber gathers locally in the vicinityof the silica particles and the terminal modified groups act effectivelyon the silica, which leads to the affinity being further enhanced andthe dispersibility of the silica being excellent. If the aromatic vinylunit content in the modified conjugated diene polymer rubber is lessthan 38% by weight, the effect of forming the fine phase-separated formfrom the other diene rubber cannot be sufficiently obtained.Additionally, the effects of increasing the rigidity and the strength ofthe rubber composition cannot be sufficiently obtained. If the aromaticvinyl unit content in the modified conjugated diene polymer rubberexceeds 48% by weight, the glass transition temperature (Tg) of theconjugated diene polymer rubber will rise, the balance betweenviscoelastic characteristics will worsen, and it will be difficult toobtain the effects of reducing heat build-up. Note that the aromaticvinyl unit content in the modified conjugated diene polymer rubber ismeasured using infrared emission spectroscopy (Hampton technique).

In the present technology, a vinyl unit content in the modifiedconjugated diene polymer rubber is from 20% to 35% by weight and ispreferably from 26% to 34% by weight. The glass transition temperature(Tg) of the modified conjugated diene polymer rubber can be madeappropriate due to configuring the vinyl unit content in the modifiedconjugated diene polymer rubber to be from 20% to 35% by weight.Additionally, in this case, the fine phase-separated form of themodified conjugated diene polymer rubber from the other diene rubber canbe stabilized. If the vinyl unit content in the modified conjugateddiene polymer rubber is less than 20% by weight, the Tg of the modifiedconjugated diene polymer rubber will decrease and the dynamicvisco-elasticity characteristic loss tangent (tan δ) at 0° C., which isthe indicator of wet performance, will decline. Moreover, in this case,the fine phase-separated form of the modified conjugated diene polymerrubber cannot be stabilized. If the vinyl unit content in the modifiedconjugated diene polymer rubber exceeds 35% by weight, there is apossibility that vulcanization rate will decline and the strength andthe rigidity will decline. Additionally, the rolling resistance cannotbe reduced. Note that the vinyl unit content in the modified conjugateddiene polymer rubber is measured using infrared emission spectroscopy(Hampton technique).

The glass transition temperature (Tg) of the modified conjugated dienepolymer rubber is from −22 to −32° C. By configuring the Tg of themodified conjugated diene polymer rubber to be from −22 to −32° C., thewet performance can be ensured and the rolling resistance can bereduced. If the Tg of the modified conjugated diene polymer rubber ishigher than −22° C., balance of the viscoelastic characteristics will benegatively affected and it will be difficult to obtain the effects ofreducing the heat build-up. If the Tg of the modified conjugated dienepolymer rubber is lower than −32° C., the Tg of the modified conjugateddiene polymer rubber will decrease and the dynamic visco-elasticitycharacteristic loss tangent (tan δ) at 0° C., which is the indicator ofgrip on wet roads, will decline. The Tg of the modified conjugated dienepolymer rubber is measured using a thermograph by differential scanningcalorimetry (DSC) at a temperature elevation speed of 20° C./minute. Thetemperature at the midpoint of the transition region is set as the glasstransition temperature thereof. Additionally, when the modifiedconjugated diene polymer rubber is an oil extended product, the glasstransition temperature is the glass transition temperature of themodified conjugated diene polymer rubber in a state where the oilextension component (the oil) is not included.

The formability/processability of a rubber composition can be enhancedby oil extending the modified conjugated diene polymer rubber. Theamount of oil extension is not particularly limited, but is preferablynot more than 25 parts by weight per 100 parts by weight of the modifiedconjugated diene polymer rubber. If the amount of oil extension of themodified conjugated diene polymer rubber exceeds 25 parts by weight, thedegree of freedom in formulation design when compounding oils,softeners, tackifiers, and the like in the rubber composition will belimited.

In the present technology, the content of the modified conjugated dienepolymer rubber is not less than 30% by weight of 100% by weight of thediene rubber, and is preferably from 40% to 90% by weight. If thecontent of the modified conjugated diene polymer rubber is less than 30%by weight of the diene rubber, affinity with the silica will worsen, anddispersibility of the silica cannot be made excellent.

In the present technology, a diene rubber other than the modifiedconjugated diene polymer rubber can be compounded as a rubber component.Examples of the other diene rubber include natural rubber, isoprenerubber, butadiene rubber, non-terminal-modified solution polymerizationstyrene butadiene rubber (S-SBR), emulsion polymerization styrenebutadiene rubber (E-SBR), butyl rubber, halogenated butyl rubber, andthe like. The other diene rubber is preferably a natural rubber, anisoprene rubber, a butadiene rubber, or an emulsion polymerizationstyrene butadiene rubber. A single rubber may be used or multiplerubbers may be blended and used as the diene rubber. A content of theother diene rubber is not more than 70% by weight of 100% by weight ofthe diene rubber, and is preferably from 10% to 60% by weight.

With the rubber composition for use in tire treads of the presenttechnology, due to the compounding of the tackifying resin, the wetperformance, particularly, the steering stability on wet road surfaces,can be further improved while maintaining the low rolling resistance.Examples of the tackifying resin include those that have a glasstransition temperature (Tg) that is from 50 to 110° C. higher than theTg of the modified conjugated diene polymer rubber. By configuring theTg of the tackifying resin to be not less than 50° C. higher than the Tgof the modified conjugated diene polymer rubber, the tan δ at 0° C. canbe increased and the wet grip performance can be enhanced. If the upperlimit of the Tg of the tackifying resin is set in excess of 110° C.higher than the Tg of the modified conjugated diene polymer rubber, theheat build-up will be negatively affected. Note that the Tg of thetackifying resin is measured according to the same method used in themeasurement of the Tg of the modified conjugated diene polymer rubberdescribed above.

The softening point of the tackifying resin is not particularly limited,but is preferably set to be from 130 to 170° C. and more preferably from140 to 165° C. If the softening point of the tackifying resin is lowerthan 130° C., the effects of improving the wet performance cannot besufficiently obtained. If the softening point of the tackifying resinexceeds 170° C., the dispersibility in the diene rubber will benegatively affected, the grip performance on wet road surfaces willdecline, and the rubber strength will decline. The softening point ofthe tackifying resin is expressed as a value measured in accordance withJapanese Industrial Standard (JIS) K6220-1 (ball and ring method).

A compounded amount of the tackifying resin is preferably from 1 to 25parts by weight and more preferably from 1 to 20 parts by weight per 100parts by weight of the diene rubber. If the compounded amount of thetackifying resin is less than 1 part by weight, the effects of improvingthe wet grip performance cannot be sufficiently obtained. If thecompounded amount of the tackifying resin exceeds 25 parts by weight,the low rolling resistance will be negatively affected. Moreover, thetackiness of the rubber composition will increase, andmoldability/processability and handling will be negatively affected dueto adhesion of the composition to the molding roller, and the like.

The type of tackifying resin used is not particularly limited, andexamples thereof include natural resins such as terpene resins, rosinresins, and the like; and synthetic resins such as petroleum resins,carboniferous resins, phenol resins, xylene resin, and the like; andmodified products thereof. Among these, terpene resins and/or petroleumresins are preferable and modified products of terpene resins are morepreferable.

Preferably, examples of the terpene resins include a-pinene resin,β-pinene resin, limonene resin, hydrogenated limonene resin, dipenteneresin, terpene phenol resin, terpene styrene resin, aromatic modifiedterpene resin, hydrogenated terpene resin, and the like. Among these,aromatic modified terpene resins are preferable, and examples thereofinclude aromatic modified terpene resins obtained by polymerizing aterpene such as α-pinene, β-pinene, dipentene, limonene, and the like,and an aromatic compound such as styrene, phenol, α-methylstyrene, vinyltoluene, and the like.

Examples of the petroleum resin include aromatic hydrocarbon resins or,alternatively, saturated or unsaturated aliphatic hydrocarbon resins.Examples thereof include C₅ petroleum resins (aliphatic petroleum resinsformed by polymerizing fractions such as isoprene, 1,3-pentadiene,cyclopentadiene, methylbutene, pentene, and the like), C₉ petroleumresins (aromatic petroleum resins formed by polymerizing fractions suchas α-methylstyrene, o-vinyl toluene, m-vinyl toluene, p-vinyl toluene,and the like), C₅C₉ copolymerization petroleum resins, and the like.

In the present technology, a compounded amount of the filler comprisingnot less than 50% by weight of silica is from 25 to 80 parts by weightand preferably from 20 to 75 parts by weight per 100 parts by weight ofthe diene rubber. By configuring the compounded amount of the filler tobe within this range, the low rolling resistance and wet performance ofthe rubber composition can be balanced at higher levels. If thecompounded amount of the filler is less than 25 parts by weight, the wetperformance cannot be ensured. If the compounded amount of the fillerexceeds 80 parts by weight, the low rolling resistance will benegatively affected.

The content of the silica in 100% by weight of the filler is not lessthan 50% by weight and is preferably from 70% to 100% by weight. Byconfiguring the content of the silica in the filler to be within thisrange, the low rolling resistance and wet performance of the rubbercomposition can be balanced at higher levels. Additionally, bycompounding the modified conjugated diene polymer rubber, affinity withthe silica is increased and dispersibility is enhanced. As a result, theeffects of compounding silica are further enhanced.

The silica may be any silica that is regularly used in rubbercompositions for use in tire treads. Examples thereof include wet methodsilica, dry method silica, surface treated silica, and the like.

With respect to the particle characteristics of the silica, the nitrogenspecific surface area (N₂SA) is preferably from 194 to 225 m²/g. TheN₂SA of the silica is calculated in accordance with JIS K6217-2.

The high specific surface area silica described above displays stronginteraction between particle surfaces and poor affinity with dienerubber. As a result, it is difficult to obtain excellent dispersibilitywhen simply compounded with diene rubber and the effects of improvingthe dynamic visco-elasticity characteristics such as the tan δ and thelike cannot be sufficiently obtained. Additionally, sufficientimprovement of the dispersibility of high specific surface area silicahas not necessarily been obtained even when compounded together with aconventional terminal-modified styrene-butadiene rubber.

In contrast, in the present technology, the dispersibility of the silicacan be improved by compounding the high specific surface area silicatogether with the modified conjugated diene polymer rubber describedabove. As such, the modified conjugated diene polymer rubber and thehigh specific surface area silica both act to improve the tan δ, whichleads to the attainment of greater synergy.

The silica to be used may be appropriately selected from commerciallyavailable products. Additionally, a silica obtained through a regularmanufacturing method may be used.

In the rubber composition of the present technology, a silane couplingagent is preferably compounded together with the silica as such willlead to an improvement in the dispersibility of the silica and a furtherincrease in the reinforcement action of the diene rubber. A compoundedamount of the silane coupling agent is preferably from 3% to 20% byweight and more preferably from 5% to 15% by weight of the compoundedamount of the silica. If the compounded amount of the silane couplingagent is less than 3% by weight of the weight of the silica, the effectof improving the dispersion of the silica cannot be sufficientlyobtained. Additionally, if the compounded amount of the silane couplingagent exceeds 20% by weight, the silane coupling agents will polymerize,and the desired effects cannot be obtained.

The silane coupling agent is not particularly limited, but is preferablya sulfur-containing silane coupling agent. Examples thereof includebis-(3-triethoxysilylpropyl)tetrasulfide,bis(3-triethoxysilylpropyl)disulfide, 3-trimethoxysilylpropylbenzothiazole tetrasulfide, γ-mercaptopropyltriethoxysilane,3-octanoylthiopropyl triethoxysilane, and the like.

The rubber composition for use in tire treads of the present technologymay also include other fillers other than the silica. Examples of suchfillers other than the silica include, carbon black, clay, mica, talc,calcium carbonate, aluminum hydroxide, aluminum oxide, titanium oxide,and the like. Among these, carbon black is preferable. This is becauserubber strength can be increased by compounding other fillers, includingcarbon black. A content of the other fillers is not more than 50% byweight and is preferably from 0% to 20% by weight of 100% by weight ofthe filler. If the content of the other fillers exceeds 50% by weight,the rolling resistance will worsen.

The rubber composition for use in tire treads may also include variouscompounding agents that are commonly used in rubber compositions for usein tire treads. Examples thereof include vulcanization or cross-linkingagents, vulcanization accelerators, antiaging agents, plasticizers,processing aids, liquid polymers, thermosetting resins, and the like.These compounding agents can be kneaded by a common method to obtain acomposition that can then be used for vulcanization or cross-linking.These compounding agents can be blended at conventional general amountsso long as the objects of the present technology are not hindered. Therubber composition for use in tire treads can be produced by mixing theabove-mentioned components using a well-known rubber kneading machinesuch as a Banbury mixer, a kneader, an open roll, or the like.

The rubber composition for use in tire treads of the present technologycan be advantageously used in pneumatic tires. The low rollingresistance and the wet performance can be enhanced to or beyondconventional levels via a pneumatic tire in which the rubber compositiondescribed above is used in the tread portion.

The present technology is further described below by examples. However,the scope of the present technology is not limited to these examples.

EXAMPLES

24 types of rubber compositions for use in tire treads were preparedaccording to the formulations shown in Tables 1 to 3 (Working Examples 1to 7 and Comparative Examples 1 to 17). The shared components shown inTable 4 (with the exception of the sulfur and the vulcanizationaccelerator) were compounded with the rubber compositions and themixtures were kneaded in a 1.8 L sealed mixed for five minutes at 160°C. Then, the mixtures were extruded as master batches, to which thesulfur and the vulcanization accelerator were added. Thereafter, themaster batches were kneaded on an open roll. Note that in Tables 1 to 3,in cases where the SBR comprises an extension oil, the compounded amountof the SBR comprising this extension oil is noted along with the NETcompounded amount of the SBR without the oil in parentheses.Additionally, the contents of the shared components shown in Table 4 areparts by weight compounded per 100 parts by weight of the diene rubbershown in Tables 1 to 3 (NET rubber content).

Vulcanized rubber samples were fabricated by pressure vulcanizing theobtained 24 types of rubber compositions for use in tire treads in amold having a predetermined shape for 20 minutes at 160° C. Then, therolling resistance (tan δ at 60° C.) of the samples was measuredaccording to the methods described below.

Rolling Resistance: tan δ (60° C.)

The rolling resistance of the obtained vulcanized rubber samples wasevaluated based on the loss tangent tan δ (60° C.), which is a knownindicator of rolling resistance. The tan δ (60° C.) was measured using aviscoelasticity spectrometer (manufactured by Toyo Seiki Seisaku-sho,Ltd.) under the following conditions: 10% initial distortion, ±2%amplitude, 20 Hz frequency, and 60° C. temperature. The results of themeasuring were indexed and recorded in Tables 1 to 3, with the indexvalue of Comparative Example 1 being 100. Smaller index values indicatesmaller tan δ (60° C.) and lower heat build-up, which in turn indicateslower rolling resistance and superior fuel consumption performance whenused in a pneumatic tire.

Next, sets of four pneumatic tires having the tire structure depicted inFIG. 1 and a tire size of 225/50R17 were fabricated. In each of the setsof four tires, one of the 24 types of rubber compositions for use intire treads described above was used in the tread portion. The steeringstability and braking performance on wet road surfaces of the resulting24 types of pneumatic tires were evaluated according to the methodsdescribed below.

Steering Stability on Wet Road Surfaces

The pneumatic tires were assembled on a wheel having a rim size of 7×J,and mounted on a 2.5 L class test vehicle (made in Japan). The pneumatictires were inflated to an air pressure of 230 kPa and the test vehiclewas driven on a 2.6 km circuit wet road surface test course. Thesteering stability while driving was scored based on sensory evaluationperformed by three experienced evaluators. The results of the measuringwere indexed and recorded in Tables 1 to 3, with the index value ofComparative Example 1 being 100. Larger index values indicate superiorwet steering stability on wet road surfaces.

Braking Performance on Wet Road Surfaces

The pneumatic tires were assembled on a wheel having a rim size of 7 xJ,and mounted on a 2.5 L class test vehicle (made in Japan). The pneumatictires were inflated to an air pressure of 230 kPa and the test vehiclewas driven on a 2.6 km circuit wet road surface test course. The brakingperformance while driving was scored based on sensory evaluationperformed by three experienced evaluators. The results of the measuringwere indexed and recorded in Tables 1 to 3, with the index value ofComparative Example 1 being 100. Larger index values indicate superiorwet braking performance on wet road surfaces.

TABLE 1 Comparative Comparative Comparative Working Example 1 Example 2Example 3 Example 1 Modified S-SBR 1 pbw 62.5 62.5 (50.0) (50.0)Modified S-SBR 2 pbw 50.0 50.0 E-SBR pbw 68.75 68.75 68.75 68.75 (50.0)(50.0) (50.0) (50.0) Silica pbw 40 40 40 40 CB pbw 5 5 5 5 Couplingagent pbw 3.2 3.2 3.2 3.2 Tackifying resin 1 pbw 5 5 Tackifying resin 2pbw Tackifying resin 3 pbw Oil pbw 2.5 5.0 5.0 2.5 tanδ (60° C.) Index100 100 97 97 value Wet steering stability Index 100 95 103 108 valueWet braking performance Index 100 95 105 111 value Working WorkingWorking Working Example 2 Example 3 Example 4 Example 5 Modified S-SBR 1pbw 62.5 62.5 62.5 62.5 (50.0) (50.0) (50.0) (50.0) Modified S-SBR 2 pbwE-SBR pbw 68.75 68.75 68.75 68.75 (50.5) (50.0) (50.0) (50.0) Silica pbw40 40 60 40 CB pbw 5 5 0 5 Coupling agent pbw 3.2 3.2 4.8 3.2 Tackifyingresin 1 pbw 10 5 Tackifying resin 2 pbw 10 Tackifying resin 3 pbw 5 Oilpbw 2.5 2.5 tanδ (60° C.) Index 98 97 98 98 value Wet steering stabilityIndex 114 119 122 125 value Wet braking performance Index 116 122 125127 value

TABLE 2 Comparative Comparative Comparative Comparative Example 4Example 5 Example 6 Example 7 Modified S-SBR 1 pbw 62.5 (50.0) ModifiedS-SBR 3 pbw 60.0 (50.0) Modified S-SBR 4 pbw 50.0 Modified S-SBR 5 pbw50.0 S-SBR pbw E-SBR pbw 68.75 68.75 68.75 68.75 (50.0) (50.0) (50.0)(50.0) Silica pbw 40 40 40 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.23.2 3.2 Tackifying resin 1 pbw 5 5 5 Tackifying resin 4 pbw 5 Oil pbw2.5 2.5 2.5 2.5 tanδ (60° C.) Index 106 100 99 101 value Wet steeringstability Index 130 100 98 102 value Wet braking Index 133 103 101 105performance value Comparative Comparative Comparative ComparativeExample 8 Example 9 Example 10 Example 11 Modified S-SBR 1 pbw 25.0 62.562.5 (20.0) (50.0) (50.0) Modified S-SBR 3 pbw Modified S-SBR 4 pbwModified S-SBR 5 pbw S-SBR pbw 68.75 (50.0) E-SBR pbw 68.75 110.0 68.7568.75 (50.0) (80.0) (50.0) (50.0) Silica pbw 40 40 40 20 CB pbw 5 5 5 25Coupling agent pbw 3.2 3.2 3.2 1.6 Tackifying resin 1 pbw 5 5 50 5Tackifying resin 4 pbw Oil pbw 2.5 2.5 2.5 tanδ (60° C.) Index 101 105116 100 value Wet steering stability Index 85 90 78 101 value Wetbraking Index 88 93 73 100 performance value

TABLE 3 Comparative Comparative Working Working Example 12 Example 13Example 6 Example 7 Modified S-SBR 1 pbw 62.5 62.5 (50.0) (50.0)Modified S-SBR 6 pbw 62.5 (50.0) Modified S-SBR 7 pbw 62.5 (50.0)Modified S-SBR 8 pbw Modified S-SBR 9 pbw Modified S-SBR 10 pbw ModifiedS-SBR 11 pbw E-SBR pbw 68.75 68.75 68.75 68.75 (50.0) (50.0) (50.0)(50.0) Silica pbw 15 80 40 40 CB pbw 3 5 5 5 Coupling agent pbw 1.2 5.63.2 3.2 Tackifying resin 1 pbw 5 5 5 5 Oil pbw 2.5 2.5 2.5 2.5 tanδ (60°C.) Index 87 110 98 98 value Wet steering stability Index 74 100 108 107value Wet braking Index 73 102 108 107 performance value ComparativeComparative Comparative Comparative Example 14 Example 15 Example 16Example 17 Modified S-SBR 1 pbw Modified S-SBR 6 pbw Modified S-SBR 7pbw Modified S-SBR 8 pbw 62.5 (50.0) Modified S-SBR 9 pbw 62.5 (50.0)Modified S-SBR 10 pbw 62.5 (50.0) Modified S-SBR 11 pbw 62.5 (50.0)E-SBR pbw 68.75 68.75 68.75 68.75 (50.0) (50.0) (50.0) (50.0) Silica pbw40 40 40 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Tackifyingresin 1 pbw 5 5 5 5 Oil pbw 2.5 2.5 2.5 2.5 tanδ (60° C.) Index 90 10590 104 value Wet steering stability Index 100 116 99 115 value Wetbraking Index 98 118 97 117 performance value

The types of raw materials used in Tables 1 to 3 are described below.

-   -   Modified S-SBR 1: Terminal-modified solution polymerization        styrene butadiene rubber prepared according to the production        method described below; modified conjugated diene polymer        rubber; aromatic vinyl unit content of 42% by weight; vinyl unit        content of 32% by weight; weight-average molecular weight (Mw)        of 750,000; Tg of −25° C.; oil extended product comprising 25        parts by weight of oil per 100 parts by weight of the rubber        component.

[Production Method of Modified S-SBR 1]

4533 g of cyclohexane, 338.9 g (3.254 mol) of styrene, 468.0 g (8.652mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and 0.189 mL (1.271mmol) of N,N,N′,N′-tetramethylethylenediamine were added to anitrogen-purged autoclave reaction vessel having an internal capacity of10 L. Then, mixing was begun. After adjusting the temperature of thematter in the reaction vessel to 50° C., 5.061 mL (7.945 mmol) ofn-butyllithium was added. After the rate of polymerization/conversionreached approximately 100%, 12.0 g more of isoprene was added and themixture was reacted for five minutes. Then, 0.281 g (0.318 mmol) of atoluene solution containing 40% by weight of1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for30 minutes. Furthermore, 18.3 g (0.318 mmol) of a xylene solutioncontaining 40% by weight of polyorganosiloxane A described below wasadded and the mixture was reacted for 30 minutes. 0.5 mL of methanol wasadded and the mixture was mixed for 30 minutes. A small amount ofantiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts ofFukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as anextension oil were added to the resulting polymer solution. Then, thesolid rubber was recovered using a steam stripping process. The obtainedsolid rubber was dehydrated using an open roll and dried in a dryer.Thus, the modified S-SBR 1 was obtained.

Polyorganosiloxane A: Polyorganosiloxane having the structure of thegeneral formula (I), wherein m=80, n=0, k=120, X¹, X⁴, R¹ to R³, and R⁵to R⁸ are each methyl groups (—CH₃), and X² is a hydrocarbon groupexpressed by formula (VIII) below.

-   -   Modified S-SBR 2: Terminal-modified solution polymerization        styrene butadiene rubber; aromatic vinyl unit content of 20% by        weight; vinyl unit content of 67%; weight-average molecular        weight (Mw) of 510,000; Tg of −25° C.; Nipol NS616 (Zeon        Corporation); non-oil extended product.    -   Modified S-SBR 3: Terminal-modified solution polymerization        styrene butadiene rubber; aromatic vinyl unit content of 35% by        weight; vinyl unit content of 48%; weight-average molecular        weight (Mw) of 450,000; Tg of −30° C.; SE0372 (manufactured by        Sumitomo Chemical Co., Ltd.); oil extended product comprising 20        parts by weight of oil per 100 parts by weight of the rubber        component.    -   Modified S-SBR 4: Terminal-modified solution polymerization        styrene butadiene rubber; aromatic vinyl unit content of 30% by        weight; vinyl unit content of 61%; weight-average molecular        weight (Mw) of 430,000; Tg of −27° C.; N207 (manufactured by        Asahi Kasei Corporation); non-oil extended product.    -   Modified S-SBR 5: Terminal-modified solution polymerization        styrene butadiene rubber; aromatic vinyl unit content of 42% by        weight; vinyl unit content of 35%; weight-average molecular        weight (Mw) of 440,000; Tg of −24° C.; Asaprene Eli)        (manufactured by Asahi Kasei Corporation); non-oil extended        product.    -   Modified S-SBR 6: Terminal-modified solution polymerization        styrene butadiene rubber prepared according to the production        method described below; modified conjugated diene polymer rubber        formed from a polyorganosiloxane having the structure of the        general formula (II); aromatic vinyl unit content of 42% by        weight; vinyl unit content of 32%; weight-average molecular        weight (Mw) of 750,000; Tg of −25° C.; oil extended product        comprising 25 parts by weight of oil per 100 parts by weight of        the rubber component.

[Production Method of Modified S-SBR 6]

4550 g of cyclohexane, 341.1 g (3.275 mol) of styrene, 459.9 g (8.502mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and 0.190 mL (1.277mmol) of N,N,N′,N′-tetramethylethylenediamine were added to anitrogen-purged autoclave reaction vessel having an internal capacity of10 L. Then, mixing was begun. After adjusting the temperature of thematter in the reaction vessel to 50° C., 5.062 mL (7.946 mmol) ofn-butyllithium was added. After the rate of polymerization/conversionreached approximately 100%, 12.0 g more of isoprene was added and themixture was reacted for five minutes. Then, 0.283 g (0.320 mmol) of atoluene solution containing 40% by weight of1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for30 minutes. Furthermore, 19.0 g (0.330 mmol) of a xylene solutioncontaining 40% by weight of polyorganosiloxane B described below wasadded and the mixture was reacted for 30 minutes. 0.5 mL of methanol wasadded and the mixture was mixed for 30 minutes. A small amount ofantiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts ofFukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as anextension oil were added to the resulting polymer solution. Then, thesolid rubber was recovered using a steam stripping process. The obtainedsolid rubber was dehydrated using an open roll and dried in a dryer.Thus, the modified S-SBR 6 was obtained.

Polyorganosiloxane B: Polyorganosiloxane having the structure of thegeneral formula (II), wherein R⁹ to R¹⁶ are each methyl groups (—CH₃),and X⁵ to X⁸ are each hydrocarbon groups expressed by the formula(VIII).

-   -   Modified S-SBR 7: Terminal-modified solution polymerization        styrene butadiene rubber prepared according to the production        method described below; modified conjugated diene polymer rubber        formed from a polyorganosiloxane having the structure of the        general formula (III); aromatic vinyl unit content of 41% by        weight; vinyl unit content of 32%; weight-average molecular        weight (Mw) of 750,000; Tg of −25° C.; oil extended product        comprising 25 parts by weight of oil per 100 parts by weight of        the rubber component.

[Production Method of Modified S-SBR 7]

4542 g of cyclohexane, 339.2 g (3.257 mol) of styrene, 462.8 g (8.556mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and 0.188 mL (1.264mmol) of N,N,N′,N′-tetramethylethylenediamine were added to anitrogen-purged autoclave reaction vessel having an internal capacity of10 L. Then, mixing was begun. After adjusting the temperature of thematter in the reaction vessel to 50° C., 5.059 mL (7.942 mmol) ofn-butyllithium was added. After the rate of polymerization/conversionreached approximately 100%, 12.0 g more of isoprene was added and themixture was reacted for five minutes. Then, 0.283 g (0.320 mmol) of atoluene solution containing 40% by weight of1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for30 minutes. Furthermore, 19.2 g (0.333 mmol) of a xylene solutioncontaining 40% by weight of polyorganosiloxane C described below wasadded and the mixture was reacted for 30 minutes. 0.5 mL of methanol wasadded and the mixture was mixed for 30 minutes. A small amount ofantiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts ofFukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as anextension oil were added to the resulting polymer solution. Then, thesolid rubber was recovered using a steam stripping process. The obtainedsolid rubber was dehydrated using an open roll and dried in a dryer.Thus, the modified S-SBR 7 was obtained.

Polyorganosiloxane C: Polyorganosiloxane having the structure of thegeneral formula (III), wherein s=2, R¹⁷ to R¹⁹ are each methyl groups(—CH₃), and X⁹ to X¹¹ are each hydrocarbon groups expressed by theformula (VIII).

-   -   Modified S-SBR 8: Terminal-modified solution polymerization        styrene butadiene rubber prepared according to the production        method described below; modified conjugated diene polymer rubber        formed from a polyorganosiloxane having the structure of the        general formula (I); aromatic vinyl unit content of 34% by        weight; vinyl unit content of 34%; weight-average molecular        weight (Mw) of 760,000; Tg of −33° C.; oil extended product        comprising 25 parts by weight of oil per 100 parts by weight of        the rubber component.

[Production Method of Modified S-SBR 8]

4541 g of cyclohexane, 277.6 g (2.665 mol) of styrene, 523.1 g (9.671mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and 0.175 mL (1.178mmol) of N,N,N′,N′-tetramethylethylenediamine were added to anitrogen-purged autoclave reaction vessel having an internal capacity of10 L. Then, mixing was begun. After adjusting the temperature of thematter in the reaction vessel to 50° C., 4.984 mL (7.824 mmol) ofn-butyllithium was added. After the rate of polymerization/conversionreached approximately 100%, 12.0 g more of isoprene was added and themixture was reacted for five minutes. Then, 0.273 g (0.327 mmol) of atoluene solution containing 40% by weight of1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for30 minutes. Furthermore, 18.1 g (0.314 mmol) of a xylene solutioncontaining 40% by weight of the polyorganosiloxane A described above wasadded and the mixture was reacted for 30 minutes. 0.5 mL of methanol wasadded and the mixture was mixed for 30 minutes. A small amount ofantiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts ofFukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as anextension oil were added to the resulting polymer solution. Then, thesolid rubber was recovered using a steam stripping process. The obtainedsolid rubber was dehydrated using an open roll and dried in a dryer.Thus, the modified S-SBR 8 was obtained.

-   -   Modified S-SBR 9: Terminal-modified solution polymerization        styrene butadiene rubber prepared according to the production        method described below; modified conjugated diene polymer rubber        formed from a polyorganosiloxane having the structure of the        general formula (I); aromatic vinyl unit content of 49% by        weight; vinyl unit content of 28%; weight-average molecular        weight (Mw) of 710,000; Tg of −17° C.; oil extended product        comprising 25 parts by weight of oil per 100 parts by weight of        the rubber component.

[Production Method of Modified S-SBR 9]

4536 g of cyclohexane, 401.0 g (3.850 mol) of styrene, 392.0 g (7.247mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and 0.201 mL (1.352mmol) of N,N,N′,N′-tetramethylethylenediamine were added to anitrogen-purged autoclave reaction vessel having an internal capacity of10 L. Then, mixing was begun. After adjusting the temperature of thematter in the reaction vessel to 50° C., 5.141 mL (8.071 mmol) ofn-butyllithium was added. After the rate of polymerization/conversionreached approximately 100%, 12.0 g more of isoprene was added and themixture was reacted for five minutes. Then, 0.279 g (0.320 mmol) of atoluene solution containing 40% by weight of1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for30 minutes. Furthermore, 18.6 g (0.323 mmol) of a xylene solutioncontaining 40% by weight of the polyorganosiloxane A described above wasadded and the mixture was reacted for 30 minutes. 0.5 mL of methanol wasadded and the mixture was mixed for 30 minutes. A small amount ofantiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts ofFukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as anextension oil were added to the resulting polymer solution. Then, thesolid rubber was recovered using a steam stripping process. The obtainedsolid rubber was dehydrated using an open roll and dried in a dryer.Thus, the modified S-SBR 9 was obtained.

-   -   Modified S-SBR 10: Terminal-modified solution polymerization        styrene butadiene rubber prepared according to the production        method described below; modified conjugated diene polymer rubber        formed from a polyorganosiloxane having the structure of the        general formula (I); aromatic vinyl unit content of 41% by        weight; vinyl unit content of 17%; weight-average molecular        weight (Mw) of 740,000; Tg of −37° C.; oil extended product        comprising 25 parts by weight of oil per 100 parts by weight of        the rubber component.

[Production Method of Modified S-SBR 10]

4542 g of cyclohexane, 339.2 g (3.257 mol) of styrene, 462.8 g (8.556mol) of butadiene, 20.0 g (0.294 mol) of isoprene and 0.0376 mL (0.253mmol) of N,N,N′,N′-tetramethylethylenediamine were added to anitrogen-purged autoclave reaction vessel having an internal capacity of10 L. Then, mixing was begun. After adjusting the temperature of thematter in the reaction vessel to 50° C., 5.059 mL (7.942 mmol) ofn-butyllithium was added. After the rate of polymerization/conversionreached approximately 100%, 12.0 g more of isoprene was added and themixture was reacted for five minutes. Then, 0.280 g (0.331 mmol) of atoluene solution containing 40% by weight of1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for30 minutes. Furthermore, 18.8 g (0.326 mmol) of a xylene solutioncontaining 40% by weight of the polyorganosiloxane A described above wasadded and the mixture was reacted for 30 minutes. 0.5 mL of methanol wasadded and the mixture was mixed for 30 minutes. A small amount ofantiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts ofFukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as anextension oil were added to the resulting polymer solution. Then, thesolid rubber was recovered using a steam stripping process. The obtainedsolid rubber was dehydrated using an open roll and dried in a dryer.Thus, the modified S-SBR 10 was obtained.

-   -   Modified S-SBR 11: Terminal-modified solution polymerization        styrene butadiene rubber prepared according to the production        method described below; modified conjugated diene polymer rubber        formed from a polyorganosiloxane having the structure of the        general formula (I); aromatic vinyl unit content of 39% by        weight; vinyl unit content of 40%; weight-average molecular        weight (Mw) of 750,000; Tg of −21° C.; oil extended product        comprising 25 parts by weight of oil per 100 parts by weight of        the rubber component.

[Production Method of Modified S-SBR 11]

4543 g of cyclohexane, 319.8 g (3.071 mol) of styrene, 480.1 g (8.876mol) of butadiene, 20.0 g (0.294 mol) of isoprene and 0.217 mL (1.462mmol) of N,N,N′,N′-tetramethylethylenediamine were added to anitrogen-purged autoclave reaction vessel having an internal capacity of10 L. Then, mixing was begun. After adjusting the temperature of thematter in the reaction vessel to 50° C., 5.141 mL (8.0714 mmol) ofn-butyllithium was added. After the rate of polymerization/conversionreached approximately 100%, 12.0 g more of isoprene was added and themixture was reacted for five minutes. Then, 0.279 g (0.320 mmol) of atoluene solution containing 40% by weight of1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for30 minutes. Furthermore, 18.6 g (0.323 mmol) of a xylene solutioncontaining 40% by weight of the polyorganosiloxane A described above wasadded and the mixture was reacted for 30 minutes. 0.5 mL of methanol wasadded and the mixture was mixed for 30 minutes. A small amount ofantiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts ofFukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as anextension oil were added to the resulting polymer solution. Then, thesolid rubber was recovered using a steam stripping process. The obtainedsolid rubber was dehydrated using an open roll and dried in a dryer.Thus, the modified S-SBR 11 was obtained.

-   -   S-SBR: Unmodified solution polymerization styrene butadiene        rubber; aromatic vinyl unit content of 41% by weight; vinyl unit        content of 25% by weight; weight-average molecular weight (Mw)        of 1,010,000; Tg of −30° C.; SLR6430 (manufactured by Dow        Chemical); oil extended product comprising 37.5 parts by weight        of oil per 100 parts by weight of the rubber component.    -   E-SBR: Emulsion polymerization styrene butadiene rubber;        aromatic vinyl unit content of 25% by weight; vinyl unit content        of 15% by weight; weight-average molecular weight (Mw) of        600,000; Tg of −52° C.; Nipol 1723 (manufactured by Zeon        Corporation); oil extended product comprising 37.5 parts by        weight of oil per 100 parts by weight of the rubber component.    -   Silica: Zeosil 1165 MP (manufactured by Rhodia); DBP absorption        number of 200 ml/100 g; nitrogen specific surface area (N₂SA) of        160 m²/g; CTAB specific surface area (CTAB) of 159 m²/g.    -   CB: Carbon black, SEAST KH (manufactured by Tokai Carbon Co.,        Ltd.)    -   Coupling agent: Si69 (manufactured by Evonik Degussa Industries)    -   Tackifying resin 1: Aromatic modified terpene resin; Tg of 31°        C.; YS Resin TO-85 (manufactured by Yasuhara Chemical Co., Ltd.)    -   Tackifying resin 2: Aromatic modified terpene resin; Tg of 53°        C.; YS Resin TO-105 (manufactured by Yasuhara Chemical Co.,        Ltd.)    -   Tackifying resin 3: Aromatic modified terpene resin; Tg of 77°        C.; YS Resin TO-125 (manufactured by Yasuhara Chemical Co.,        Ltd.)    -   Tackifying resin 4: Phenol-modified terpene resin; Tg of 87° C.;        YS Polyster T145 (manufactured by Yasuhara Chemical Co., Ltd.)    -   Oil: Extract No. 4S (manufactured by Showa Shell Seikyu K.K.)

TABLE 4 Common Formulation of the Rubber Compositions Zinc oxide   3parts by weight Stearic acid   2 parts by weight Antiaging agent   2parts by weight Wax   2 parts by weight Sulfur 1.8 parts by weightVulcanization   2 parts by weight accelerator 1 Vulcanization 1.5 partsby weight accelerator 2

The types of raw materials used in Table 4 are described below.

-   -   Zinc oxide: Zinc Oxide #3 (manufactured by Seido Chemical        Industry Co., Ltd.)    -   Stearic acid: Beads Stearic Acid YR (manufactured by NOF Corp.)    -   Antiaging agent: Santoflex 6PPD (manufactured by Flexsys)    -   Wax: SANNOC (manufactured by Ouchi Shinko Chemical Industrial)    -   Sulfur: “Golden Flower” oil-treated sulfur powder (manufactured        by Tsurumi Chemical Industry Co., Ltd.)    -   Vulcanization Accelerator 1: Vulcanization accelerator CBS;        Nocceler CZ-G (manufactured by Ouchi Shinko Chemical Industrial        Co., Ltd.)    -   Vulcanization Accelerator 2: Vulcanization accelerator DPG;        Nocceler D (manufactured by Ouchi Shinko Chemical Industrial        Co., Ltd.)

As is clear from Tables 1 and 3, with the rubber compositions for use intire treads of Working Examples 1 to 7, enhanced low rolling resistance(tan δ at 60° C.), wet steering stability, and wet braking performancewere confirmed. With the rubber composition of Comparative Example 2,the wet steering stability and the wet braking performance declinedbecause, in contrast with Comparative Example 1, the tackifying resin 1was not compounded. With the rubber composition of Comparative Example3, compared to Working Examples 1 to 5, the wet steering stability andthe wet braking performance were inferior because, in contrast withWorking Examples 1 to 5, the tackifying resin was not compounded.

As is clear from Table 2, with the rubber composition of ComparativeExample 4, the rolling resistance is negatively affected because the Tgof the tackifying resin 4 is more than 110° C. higher than the Tg of themodified S-SBR 1. With the rubber composition of Comparative Example 5,the rolling resistance cannot be reduced and the wet steering stabilityand the wet braking performance cannot be improved because the aromaticvinyl unit content in the modified S-SBR 3 is less than 38% by weight,the vinyl unit content exceeds 35% by weight, and the weight-averagemolecular weight is less than 600,000. With the rubber composition ofComparative Example 6, the rolling resistance cannot be reduced and thewet steering stability and the wet braking performance cannot beimproved because the aromatic vinyl unit content in the modified S-SBR 4is less than 38% by weight, the vinyl unit content exceeds 35% byweight, and the weight-average molecular weight is less than 600,000.With the rubber composition of Comparative Example 7, the rollingresistance cannot be reduced and the wet steering stability and the wetbraking performance cannot be improved because the weight-averagemolecular weight of the modified S-SBR 5 is less than 600,000. With therubber composition of Comparative Example 8, the rolling resistancecannot be reduced and the wet steering stability and the wet brakingperformance are negatively affected because the unmodified S-SBR iscompounded in place of the modified conjugated diene polymer rubber.With the rubber composition of Comparative Example 9, the rollingresistance cannot be reduced and the wet steering stability and the wetbraking performance decline because the content of the modified S-SBR 1is less than 30% by weight in the diene rubber. With the rubbercomposition of Comparative Example 10, the rolling resistance, the wetsteering stability, and the wet braking performance are negativelyaffected because the compounded amount of the tackifying resin 1 exceeds25 parts by weight.

As is clear from Table 3, with the rubber composition of ComparativeExample 11, the rolling resistance cannot be reduced and the effects ofimproving the wet steering stability and the wet braking performancecannot be obtained because the content of the silica in the filler isless than 50% by weight. With the rubber composition of ComparativeExample 12, the wet steering stability and the wet braking performancedecline because the compounded amount of the filler comprising thesilica is less than 25 parts by weight. With the rubber composition ofComparative Example 13, the rolling resistance cannot be reduced, andthe effects of improving the wet steering stability and the wet brakingperformance are insufficient because the compounded amount of the fillercomprising the silica exceeds 80 parts by weight.

With the rubber composition of Comparative Example 14, the effects ofimproving the wet steering stability and the wet braking performance areinferior compared to those of Working Examples 1 to 5 because thearomatic vinyl unit content in the modified S-SBR 7 is less than 38% byweight. With the rubber composition of Comparative Example 15, the glasstransition temperature (Tg) of the conjugated diene polymer rubber risesand the rolling resistance cannot be reduced because the aromatic vinylmonomer unit content in the modified S-SBR 9 is greater than 48% byweight. With the rubber composition of Comparative Example 16, theeffects of improving the wet steering stability and the wet brakingperformance are inferior compared to those of Working Examples 1 to 7because the vinyl unit content in the modified S-SBR 10 is less than 20%by weight. With the rubber composition of Comparative Example 17, therolling resistance cannot be reduced because the vinyl unit content inthe modified S-SBR 11 exceeds 35% by weight.

1. A rubber composition for use in tire treads comprising a diene rubberincluding not less than 30% by weight of a modified conjugated dienepolymer rubber and, per 100 parts by weight of the diene rubber, from 1to 25 parts by weight of a tackifying resin, and from 25 to 80 parts byweight of a filler; the filler comprising not less than 50% by weight ofsilica; the modified conjugated diene polymer rubber being obtained bycopolymerizing a conjugated diene monomer unit and an aromatic vinylmonomer unit in a hydrocarbon solvent using an organic active metalcompound as an initiator; a resulting active conjugated diene polymerchain thereof having a terminal modified group, obtained by reacting theactive terminal of the polymer chain with at least one type of compoundhaving a functional group that is reactable with the active terminal ofthe polymer chain; the terminal modified group having a functional groupthat interacts with the silica; in the modified conjugated diene polymerrubber, aromatic vinyl unit content being from 38% to 48% by weight,vinyl unit content being from 20% to 35%, weight-average molecularweight being from 600,000 to 1,000,000, and glass transition temperaturebeing from −22 to −32° C.; and a glass transition temperature of thetackifying resin being from 50 to 110° C. higher than the glasstransition temperature of the modified conjugated diene polymer rubber.2. The rubber composition for use in tire treads according to claim 1,wherein the compound comprising the functional group that is reactablewith the active terminal of the active conjugated diene polymer chaincomprises at least one type of polyorganosiloxane compound selected fromgeneral formulae (I) to (III) below:

wherein R¹ to R⁸ are identical or different and are alkyl groups havingfrom 1 to 6 carbons or aryl groups having from 6 to 12 carbons, X¹ andX⁴ are identical or different and are groups having functional groupsthat react with the active terminal of the active conjugated dienepolymer chain, alkyl groups having from 1 to 6 carbons, or aryl groupshaving from 6 to 12 carbons, X² is a group having a functional groupthat reacts with the active terminal of the active conjugated dienepolymer chain, X³ is a group including from 2 to 20 repeating alkyleneglycol units, a portion of the X³ moieties optionally being groupsderived from groups including from 2 to 20 repeating alkylene glycolunits, and m is an integer from 3 to 200, n is an integer from 0 to 200,and k is an integer from 0 to 200;

wherein R⁹ to R¹⁶ are identical or different and are alkyl groups havingfrom 1 to 6 carbons or aryl groups having from 6 to 12 carbons, and X⁵to X⁸ are groups having functional groups that react with the activeterminal of the active conjugated diene polymer chain;

wherein R¹⁷ to R¹⁹ are identical or different and are alkyl groupshaving from 1 to 6 carbons or aryl groups having from 6 to 12 carbons,X⁹ to X¹¹ are groups having functional groups that react with the activeterminal of the active conjugated diene polymer chain, and s is aninteger from 1 to
 18. 3. A pneumatic tire comprising the tire tread-userubber composition described in claim
 1. 4. A pneumatic tire comprisingthe tire tread-use rubber composition described in claim
 2. 5. Therubber composition for use in tire treads according to claim 1, whereinthe compound comprising the functional group that is reactable with theactive terminal of the active conjugated diene polymer chain comprisesat least one type of polyorganosiloxane compound according to generalformula (I) below:

wherein R¹ to R⁸ are identical or different and are alkyl groups havingfrom 1 to 6 carbons or aryl groups having from 6 to 12 carbons, X¹ andX⁴ are identical or different and are groups having functional groupsthat react with the active terminal of the active conjugated dienepolymer chain, alkyl groups having from 1 to 6 carbons, or aryl groupshaving from 6 to 12 carbons, X² is a group having a functional groupthat reacts with the active terminal of the active conjugated dienepolymer chain, X³ is a group including from 2 to 20 repeating alkyleneglycol units, a portion of the X³ moieties optionally being groupsderived from groups including from 2 to 20 repeating alkylene glycolunits, and m is an integer from 3 to 200, n is an integer from 0 to 200,and k is an integer from 0 to
 200. 6. The rubber composition for use intire treads according to claim 1, wherein the compound comprising thefunctional group that is reactable with the active terminal of theactive conjugated diene polymer chain comprises at least one type ofpolyorganosiloxane compound according to general formula (II) below:

wherein R⁹ to R¹⁶ are identical or different and are alkyl groups havingfrom 1 to 6 carbons or aryl groups having from 6 to 12 carbons, and X⁵to X⁸ are groups having functional groups that react with the activeterminal of the active conjugated diene polymer chain.
 7. The rubbercomposition for use in tire treads according to claim 1, wherein thecompound comprising the functional group that is reactable with theactive terminal of the active conjugated diene polymer chain comprisesat least one type of polyorganosiloxane compound according to generalformula (III) below:

wherein R¹⁷ to R¹⁹ are identical or different and are alkyl groupshaving from 1 to 6 carbons or aryl groups having from 6 to 12 carbons,X⁹ to X¹¹ are groups having functional groups that react with the activeterminal of the active conjugated diene polymer chain, and s is aninteger from 1 to 18.